The present invention relates to a method for effecting digital/analog conversion of PCM signals by a generalized interpolation.
Presently available digital/analog converters with high amplitude resolution of up to n=18 bits per sample operate according to the principle of weighted current sources, as described in the publication by Seitzer, D: "Elektronische Analog-Digital-Umsetzer" [Electronic Analog/Digital Converters] published by Springer Verlag in the Federal Republic of Germany, in 1977. The simple digital portion of the converter essentially includes registers for storing the respectively applicable code word at the input of the converter. The more complicated analog portion of a digital/analog converter with an amplitude resolution corresponding to n bits per sample is composed of n connected precision current sources whose weighted currents are combined at the output of the converter. The weighting of the currents is effected to correspond to the value of the bits switching the input.
The manufacture of a digital/analog converter with weighted current sources, for an amplitude resolution corresponding to n bits per sample, requires the use of components having a tolerance of 1 part in 2.sup.n. For example, for a 16-bit digital/analog converter this tolerance is 1 part in 65,536. When constructing such converters, the requirements for precision of this order of magnitude can be met only with the use of expensive precision components and by the implementation of a cost-intensive matching procedure. A uniformly high amplitude resolution, e.g. of 16 bits per sample, after long periods of operation can be assured only by repeatedly renewed matching.
Upon a change in the PCM code words applied to the input, not all weighted current sources will switch at exactly the same moment so that for a short time undefined, peak-like currents flow at the output of the digital/analog converter. These switching peaks, also called spikes or glitches, must be suppressed by complicated follower circuits since otherwise, for example if the converters are used in high quality PCM audio systems, they would lead to noticeable reductions in fidelity.
An interpolative method for digital/analog conversion of PCM signals has been proposed by Ritchie, G. R., Candy, J. C. and Ninke, W. H., in the article "Interpolative Digital to Analog Converters", published in IEEE Transactions on Communications, November 1974. Here each PCM code word of a length of n bits present at the input is split into two parts. The higher valued, or weighted, code word part, of a length of k bits, is switched to an adder whose outputs are connected with a digital/analog converter constructed to produce 2.sup.k +1 analog representative values. The remaining code word part, of a length of m bits and a lower value, or weight, is switched to an accumulator composed of a register and a binary adder and which operates at a clock pulse frequency f.sub.S which is greater by the factor N=2.sup.m than the PCM sampling frequency f.sub.A. The accumulator repeatedly effects binary addition of the m-bit word part to the m least significant bits of the result of the preceding addition. The carries, i.e. the bits in the (m+1)th bit position, are added to the higher valued code word part of the length of k bits and are thus considered in the subsequently connected digital/analog converter.
The operating principle of such a system is shown in FIGS. 1a and 1b, which illustrate the principle of an interpolative digital/analog converter according to Ritchie et al, supra. FIG. 1a shows the basic circuit arrangement while FIG. 1b shows the waveform of the D/A converter output over one sampling period, 1/f.sub.A. For the illustrated example, n=8 and k=m=4. The higher valued 4-bit code word part is initially used to preselect a representative amplitude value for the output of the digital/analog converter. Controlled by the transmitted signal from the accumulator, which includes the register clocked at a frequency Nf.sub.A and a binary adder connected in series therewith, switching is effected between the preselected representative value and the next higher representative amplitude value in a pattern determined by the value of the k-bit part so that over the sampling interval 1/f.sub.A the information of the lower valued 4-bit code word part determines the average value of the analog output signal. The time averaging is effected by a lowpass filter connected in series with the output of the digital/analog converter. This lowpass filter is required there in any event in order to suppress the periodic continuations of the converted useful signal spectrum above half the sampling rate W=f.sub.A /2.
The advantage of the method disclosed by Ritchie et al, supra, for the interpolative digital/analog conversion of PCM code words of a length of n bits lies in the reduction of the number of analog representative values required in the converter from 2.sup.n to 2.sup.k +1. The requirements for linearity of the converter, however, remain unchanged and high. If, moreover, the switching frequency of the converter is considered, which is increased by the factor N=2.sup.m, the method disclosed by Ritchie et al, supra, does not produce a noticeable advantage over the digital/analog conversion with weighted current sources, at least not for high amplitude resolution, for example, that corresponding to 16 bits per sample.
If the sampling rate required for PCM audio systems is placed between 32 and 50 kHz, a 16-bit converter will produce, in view of the clock pulse frequency ratio N, accumulator clock pulse frequencies in the GHz range. At the stated sampling rates, the above-described method for interpolative digital/analog conversion for high amplitude resolution is evidently unsuitable, and this has also been noted by Ritchie et al.